Numerical investigation and extensive parametric analysis of cryogenic latent heat shell and tube thermal energy storage system

Latent heat thermal energy storage systems have shown promising energy management solutions in different domains due to their high efficiency and energy density. Nowadays, most studies focus on moderate to high operating temperature applications such as solar power plants, while few studies exist for cryogenic applications such as the liquefied natural gas (LNG) domain. This article aims to assess the thermal performance of a shell and tube latent heat thermal energy storage system. The purpose of this system is to reliquefy the excess boil-off gas (BOG) generated on LNG carriers, using n-Butane as a suitable phase change material (PCM). The system is numerically investigated using Dymola software, where the solidification and melting evolution of the PCM is accounted for by using the enthalpy method and applying the finite volume method discretization technique. This system works on two operating modes: charging and discharging. During charging, the PCM solidifies and stores the cold energy during the LNG evaporation to be discharged at later times to condense the BOG while the PCM melts. An extensive parametric study is carried out to assess the impact of different factors on the system's thermal performance. Results showed that the existence of natural convection would significantly enhance the PCM performance. Moreover, the selection of the tube diameter, PCM thickness, and the heat transfer fluid mass flow rate is crucial and is a compromise of several factors. On the other hand, radial and longitudinal fins were assessed as heat transfer enhancement techniques, where a parametric study on the fins' volume, thickness, and number is carried out. The excessive addition of fins is expected to obstruct the natural convection forces; however, a considerable enhancement in the thermal performance, especially for the radial ones, is observed.